Recently, the demand for high capacity and high power electrochemical devices is gradually increasing in small polymer and medium and large electrochemical device markets. A thin-film separator suitable for the design of high capacity and high power electrochemical devices needs to have low electrical resistance and maintain stability.
A substrate for use in a separator may be greatly classified into three in an aspect of its manufacturing methods: first is a method by which a nonwoven fabric type porous substrate is made from thin fibers produced from polyolefin or the like, second is a dry method by which a thick polyolefin film is made and then stretched at a low temperature to cause micro cracks to occur between crystalline domains of polyolefin, called lamellae, to form micro pores, and third is a wet method by which polyolefin and a diluent are blended at a high temperature into a single phase, and after phase separation of the polyolefin and the diluent during cooling, the diluent is extracted to form pores in the polyolefin.
The porous substrate (porous polymer film) manufactured in this way has comparatively poor thermal and mechanical properties, and thus, with an aim to enhance the safety, a porous coating layer including inorganic particles or organic particles and binder polymer is coated on the porous substrate to manufacture a heat resistant composite separator. The binder polymer serves to bind the inorganic particles or organic particles, but as the content of the binder polymer increases, a final product, i.e., the separator, increases in permeation time and electrical resistance increases, resulting in a deterioration in the performance of an electrochemical device.
Particularly, polymer and medium and large electrochemical devices include binder polymer with high content to enhance the adhesive strength between an electrode and a composite separator, and in this case, as the binder polymer content increases, a permeation time and electrical resistance of a final product, a separator, increases.
Therefore, there is a need for technology that allows binder polymer to sufficiently exhibit its unique function without degrading the battery performance such as a permeation time or electrical resistance even if the binder polymer is used in a large amount.